1. Field of the Invention
This invention relates to a non-destructive measuring or testing method for ascertaining the state of discharge of a battery. It also relates to apparatus or an instrument for carrying out the method.
Electrically, a battery is characterized by the open circuit voltage between its terminals, by its internal resistance, and by its capacity. The latter, usually expressed in ampere-hours (Ah) or milliampere-hours (mAh), indicates how much current the battery can supply over a given time before its voltage under an appropriate load drops below a predetermined level.
For a used battery, the ratio between the Ah it has supplied to a load and its nominal capacity defines the state of discharge of the battery, this state being referred to as D and being usually expressed in %.
While the open circuit voltage of a battery of a given type is a known and stable parameter, there may be some discrepancy between its capacity and the nominal value. As for the state of discharge of a battery, this depends on the way in which it was used or even stocked, i.e. on its history. Indeed, even while being stocked at ambient temperature a battery has a tendency to lose its charge, so that its state of discharge cannot therefore be precisely known, except by measurement.
2. Prior Art
The known methods for determining the state of discharge D of a battery unfortunately are difficult to carry out and are inaccurate. They are mainly based either on the measurement of the resistance R or of the complex impedance Z of the battery, or on the measurement of the voltage E under a load, i.e. when the battery is supplying a current i. These are in fact the only parameters that vary significantly with the state of discharge of the battery.
For example, one method of measuring D is to cause the battery to supply progressively increasing current. The voltage reached by the battery after a given time or for a given current, or the value of its d.c. resistance, given by the tangent to the curve E=f(i), can be taken as representative values of the state of discharge of the battery, as described in J. J. Winter et al's article in the Journal of the Electrochemical Society, Vol. 122 (1975) pages 1434-1438 or in Froidevaux's U.S. Pat. No. 4,204,162 of May 1980. This method, besides being complicated to carry out, has also proved to be somewhat inaccurate.
A galvanostatic method has been proposed by F. Kornfeil in an article entitled "Some observations on the prediction of the state of discharge of batteries" in the Journal of the Electrochemical Society, Vol 123 (1976) pages 1271-1276. It consists in causing the batter to supply a constant current and in taking its voltage after a predetermined time as representative of its state of discharge. The results obtained by this method of measurement are satisfactory. However, the resistance of the battery, which depends on its state of discharge and varies from one sample to the next, causes the voltage to drop when the direct current is applied and thus produces a scattering in the measurements. An improved measuring method has been proposed by the applicant in French Patent Application No. 83.04430 wherein the resistance of the battery no longer plays a part in determining its state of discharge. This method usually provides good results, but in certain cases lacks accuracy when measuring low values of D.
Finally, various methods of measuring D based on the use of certain parameters of the internal complex impedance Z of the battery have been described in many publications.
For example, in an article by S. Sathyanarayana et al entitled "Impedance parameters and the state-of-charge. I. Nickel-cadmium battery" in the Journal of Applied Electrochemistry, Vol. 9 (1979) pages 125-139, the authors show that the argument a of complex impedance Z, when measured at an adequate frequency, with a also being the phase difference angle between the voltage across the terminals of impedance Z and the current flowing through it, is a parameter whose value is closely related to the state of discharge D of the battery. With this method however, the internal ohmic resistance of the battery, which depends on its state of discharge and may vary from one sample to the next, diminishes the accuracy of the measurements.
The end of vector Z, which represents the internal impedance of the battery, roughly follows an arc of a circle in a complex plane when the measurement frequency f varies approximately from 10 Hz to 10 kHz. The diameter d of the corresponding circle may also be used as a parameter representative of D, as described in S.A.G.R. Karunathilaka et al's article entitled "The impedance of the alkaline zinc-mercuric oxide cell. I. Cell behaviour and interpretation of impedance spectra" in the Journal of Applied Electrochemistry, Vol. 11 (1981) pages 573-582. This method, although it provides accurate results, has a drawback in that many measuring points are needed to determine D.
Argument a passes through a minimum at a frequency f.sub.min when impedance Z is measured within a frequency band ranging from 0.01 Hz to 10 kHz, as described in S.A.G.R. Karunathilaka et al's article "A state-of-charge test for the Li-CuO cell" in the Journal of Applied Electrochemistry, Vol. 13 (1983) pages 351-354. This frequency f.sub.min, which varies according to the state of discharge of the battery, may also be used as a parameter for measuring D. The determination of f.sub.min, however, requires a large number of measuring points.
As a final example, a method has been proposed wherein the real part R of impedance Z, measured at a given frequency, is used as a parameter representative of D. This method is described in S.A.G.R. Karunathilaka et al's article entitled "The prediction of the state-of-charge of some commercial primary cells" in the Journal of Applied Electrochemistry, Vol. 13 (1983) pages 577-586. The main drawback of this method is the difficulty in finding a frequency at which the correlation between R and the state of discharge is satisfactory. Also, the ohmic resistance of the battery has an influence on and diminishes the accuracy of the measurements.